Colicin E5

Colicin E5 is a type of Colicin, a bacteriocin made by E. coli which acts against other nearby E. coli to kill them with its tRNase activity; it digests specific tRNAs, inhibiting protein synthesis and leading to the death of the cell. ColE5 is therefore an RNA restriction enzyme.

Synthesis and release
The plasmid containing the gene encoding ColE5 also encodes its Colicin Immunity Protein, ImmE5. This binds to the cytotoxic domain of ColE5 and inhibits its tRNase activity while in the colicinogenic cell. The structure shows this cytotoxic domain bound to the Imm5 protein.

Mechanism of uptake
ColE5 binds to the vitamin B12 outer membrane receptor BtuB on the target cell via its central receptor binding domain. This triggers the recruitment of OmpF and the TolQRAB complex, which together allow the translocation of the cytotoxic domain of the colicin across the membrane and into the cytoplasm. The mechanism by which this occurs is currently unknown.

Killing Activities
ColE5 specifically cleaves the anticodons of the tRNAs for tyr, his, asn and asp, which each share the unique sequence UQU in the loop. This activity occurs in the 115 residue C terminal domain, and the loop is cleaved between the Q (modified G) and the 3' U, which corresponds to the first and second letters of the anticodon triplet. This specificity is determined by a code YGUN surrounding the anticodon. It also cleaves the same tRNAs as above when they lack the Q, so the base modification unique to ColE5 sensitive tRNAs does not appear to be recognised by ColE5. This structure shows the tRNase domain of ColE5.